If a screw is torqued to 7 in-lbs torque, that means that, if you used a wrench 7 inches long, and pushed with 1 lbs of force at the end of that wrench, you'd achieve that torque.

By tightening with different torque, you're achieving a larger preload on the bolt.

If you look at that picture, the bolt acts like a spring. Bolt head and nut are in fixed position (to simplify), but as you tighten the bolt, the bolt threads "travel" down the nut so the bold stretches just like a spring would. The elastic force of that stretch keeps the two plates together. The tighter you turn the bolt, the more you stretch it, the more preload you get, and ultimately the more clamping force there is.